Computers often communicate over a network using a modem at a subscriber's location. This modem facilitates the communication of data from the subscriber to a remote location, such as to a central office of a service provider. The modem typically receives power from an external power supply. When the modem loses power, the modem generally informs the central office or other remote location of the power loss. This allows the central office to stop transmitting data to the modem when the modem is unable to receive that data.
Modems often use one or more capacitors to store a voltage charge from the power supply. To determine when a power loss occurs, modems typically use power loss circuitry coupled to the capacitors. The power loss circuitry operates to monitor the voltage between the capacitors and the power supply. When the modem loses power, the power loss circuitry generally detects only the voltage stored in the capacitors. As the capacitors discharge, the charge stored in the capacitors falls below a threshold value, often called a “trip point.” The power loss circuitry then generates an output signal indicating that a power loss has occurred. Because the power loss circuitry monitors the level of the voltage charge stored in the capacitors, the circuitry is often referred to as a “level detector.”
A problem with this approach is that a relatively long period of time may elapse between the time the modem loses power and the time the power loss circuitry detects the power loss. The power loss circuitry typically must wait for the capacitors to discharge enough voltage before sensing the power loss. This increases the time between the power loss and the detection of the power loss, and this period of time is often important. For example, the power loss circuitry needs to detect the power loss while enough internal power remains in the modem to transmit a power loss signal to the central office. If too much time elapses before the circuitry detects the power loss, the modem will not have enough power to generate and transmit the power loss signal.
Another problem with this approach is that it typically increases the amount of circuitry used in the modem. For example, this approach often requires the addition of several components, such as additional capacitors coupled to the power supply. These additional components increase the overall cost of the modem and take up space that could be used for other elements in the modem.